The present invention relates to a process for producing dimethyl sulfoxide (hereinafter called DMSO) industrially widely used, for example, as a reaction solvent of medical and agricultural intermediate products, a synthetic reagent or as a special detergent for electronic materials, and the like. In more detail, the present invention relates to an improvement for the process of continuously oxidizing dimethyl sulfide (hereinafter called DMS) using NOx (a mixture of NO, N2O3, NO2 or N2O4 or mixture thereof) as a catalyst in a liquid phase for producing DMSO.
As a conventional process for producing DMSO, a process of continuously oxidizing DMS using NOx as a catalyst in a liquid phase is publicly known (U.S. Pat. No. 2,825,744 and Japanese Patent Publication (Kokoku) No. Sho 42-9771).
In the DMS oxidizing reaction, NOx gas to be used as a catalyst is supplied together with a gas mainly composed of oxygen to be used as an oxidizing agent to a DMS-containing liquid phase reaction system, and is released outside the reaction system as a reaction off gas, usually to be treated by an alkali for disposal. The amount of NOx gas used is as much as required as a catalyst for DMS. In the oxidation reaction, new NOx is kept supplied as a catalyst to the reaction system, and the used catalyst NOx is disposed of. So, since the conventional process requires a considerable amount of catalyst NOx, the disposal of used catalyst NOx is undesirable in view of cost and effective utilization of a resource.
Furthermore, the amount of oxygen gas used as an oxidizing agent in this process is generally an approximately theoretical amount for liquid phase DMS or slightly larger than it. However, the highly pure oxygen gas with an oxygen concentration of 99% or more used here is high in purchase price, and requires large equipment such as a receiving tank for use of oxygen.
An object of the present invention is to provide an efficient DMSO production process improved in the conversion from DMS to DMSO in the continuous oxidation reaction of DMS for producing DMSO by recycling NOx used as a catalyst, in order to improve the disadvantage of having to dispose of the used catalyst in the prior art. More specifically, it is an object to provide a DMSO production process improved in the recovery rate and/or absorption rate of NOx by recovering NOx from the reaction off gas and/or the gas removed from the reaction product solution, in the reaction to continuously oxidize DMS by a gas mainly composed of oxygen, using NOx as a catalyst in a liquid phase for producing DMSO.
Another object of the present invention is to provide a low-cost DMSO production process allowing the use of simpler production equipment, by using relatively low pure oxygen gas obtained by the pressure swing adsorption treatment of air.
Other objects of the present invention will be apparent from the following description.
The inventors studied intensively to solve the above problems and, as a result, found that dimethyl sulfoxide (DMSO) can be produced very efficiently at a high yield, by supplying at least partially the reaction product solution containing dimethyl sulfoxide (DMSO) and NOx obtained by oxidation reaction of dimethyl sulfide (DMS) to said oxidation reaction for recycling.
The process for producing dimethyl sulfoxide of the present invention, in which dimethyl sulfide is continuously oxidized by a gas mainly composed of oxygen using NOx as a catalyst in a liquid phase, comprises the step of supplying at least partially the reaction product solution containing dimethyl sulfoxide and NOx obtained by said oxidation reaction, to the oxidation reaction for recycling. More specifically, the present invention relates to a process for producing dimethyl sulfoxide, in which dimethyl sulfide is continuously oxidized by a gas mainly composed of oxygen using NOx as a catalyst in a liquid phase using an oxidation reactor, comprising the steps of causing the dimethyl sulfoxide solution to absorb the NOx contained in the reaction off gas in an absorbing column, and supplying the NOx-containing solution to the oxidation reactor for recycling. It is preferable to use the oxygen gas obtained by the pressure swing adsorption treatment of air described later, as the gas mainly composed of oxygen.
The DMSO production process of the present invention also includes the following preferred embodiments.
(1) Nitrogen gas is blown into the reaction product solution at a temperature higher than the reaction temperature, for bringing nitrogen into contact with the reaction product solution, to remove NOx from the reaction product solution mainly composed of NOx and DMSO.
(2) Reaction off gas and/or the gas mainly composed of NOx removed from the reaction product solution is sent to a NOx absorbing column, for recovering NOx.
(3) The reaction product solution remaining after removing NOx is used as the NOx absorbable solution.
(4) NO in the reaction off gas and/or the gas removed from the reaction product solution is oxidized into NO2 which is then sent to a NOx absorbing column, for recovering NOx.
(5) A gas with an oxygen concentration of about 80 to about 96% obtained by pressure swing adsorption treatment is used.
The drawing show the outline of the recovering apparatus used for recovery of NOx in the present invention.
The present invention is described below in more detail.
The present invention relates to an improvement of the process for continuously oxidizing DMS by a gas mainly composed of oxygen using NOx as a catalyst in a liquid phase for producing DMSO. The present invention includes several preferable conditions as described below for improving the conversion from DMS to DMSO in oxidation reaction, and for improving the recovery rate and/or absorption rate of NOx used as a catalyst, and also for allowing the use of a low pure oxygen gas as an oxidizing agent.
The oxidation reaction of DMS in the present invention is exothermic. So, though various types of oxidation reactors are available for the oxidation reaction of DMS, a highly effective mixing reactor and a multitubular reactor can be preferably used. Especially a highly effective mixing reactor can be preferably used.
In the present invention, it is preferable that the temperature in the oxidation reactor is about 10xc2x0 C. to about 50xc2x0 C. A more preferable range is about 20xc2x0 C. to about 40xc2x0 C. If the temperature is lower than about 10xc2x0 C. the produced DMSO tends to be coagulated, thereby not allowing the reaction to continue. If higher than about 50xc2x0 C. dimethyl sulfone (hereinafter called DMSO2) produced by further oxidizing DMSO is produced as a byproduct in a large amount.
It is preferable that the pressure in the reactor during the oxidation reaction is around atmospheric pressure, but the reaction can take place even at a higher pressure.
It is preferable that the catalyst NOx concentration used in the DMS oxidizing reaction in the present invention is about 0.01 to about 0.2 as NOx/DMS ratio by weight. A more preferable range is about 0.05 to about 0.12 as the NOx/DMS ratio by weight. If the reaction is effected at a NOx/DMS ratio by weight of lower than about 0.01, the conversion into DMSO declines disadvantageously. If the reaction is effected at a NOx/DMS ratio by weight of higher than about 0.2, DMSO2 tends to be produced in a large amount.
Furthermore, in the present invention, it is preferable that the oxygen concentration of the gas mainly composed of oxygen (oxygen gas) used as an oxidizing agent is about 80 wt % or more. If a gas with an oxygen concentration of lower than about 80 wt % is used as an oxidizing agent, the conversion into DMSO tends to decline. To keep the conversion high, the reaction solution is retained for a period of time corresponding to more than twice that of an ordinary case.
As the oxygen gas, a highly pure oxygen gas with an oxygen concentration of about 99 wt % or more can also be used, but in the present invention, a relatively low pure oxygen gas obtained by pressure swing adsorption treatment (hereinafter called xe2x80x9cthe PSA processxe2x80x9d) can also be used.
The PSA process refers to a method of enhancing the oxygen concentration in a gas by using the difference between oxygen and nitrogen in adsorbing power when the gas is adsorbed by an adsorbent with the rise of pressure and desorbed with the lowering of pressure. As the adsorbent, mainly zeolite is used. In this PSA process, the oxygen concentration in the gas obtained with air as the raw material can be freely adjusted in a range from about 21 wt % to about 96 wt %. In the present invention, an oxygen gas with an oxygen concentration of about 80 wt % to about 96 wt % obtained by treating air by the PSA process can be used.
In the present invention, for efficient oxidation of DMS, it is preferable that the amount of the oxygen gas supplied to the oxidation reaction system is more than the theoretical amount, and the extra unreactive oxygen gas is released outside the reaction system as a reaction off gas together with NOx used as a catalyst. In this case, the NO concentration in the NOx in the reaction off gas is usually about 5 wt % to about 60 wt %, though depending on the process conditions.
The oxygen concentration in the reaction off gas is usually about 10 wt % to about 50 wt %, though also depending on the reaction conditions. It is preferable that the oxygen gas is supplied into the oxidation reactor by an amount slightly larger than the theoretical amount as described above, and it is preferable that the oxygen concentration in the reaction off gas is about 20 wt % or more.
Moreover, to raise the NOx absorption efficiency in the present invention, it is preferable that NO in the reaction off gas is oxidized into NO2 in the gas phase NO oxidizing reactor by oxygen in the mixed gas consisting of NOx and oxygen released outside the liquid phase reactor. Furthermore, in the present invention, to raise the NO oxidation efficiency, it is preferable that a cooler is attached between liquid phase reactor and NO oxidizing reactor. Preferable temperature in the NO oxidizing reactor is lower than about 50xc2x0 C. A more preferable temperature is lower than about 30xc2x0 C. So, though various types of reactors are available for the oxidation reaction of NO, a tubular reactor is preferred. It is preferable that the pressure in the reactor during the oxidation reaction is atmospheric pressure, but the reaction can take place even at a higher pressure. Preferable NO concentration in the NOx is less than about 5 wt %. So, it is preferable that the oxygen concentration in the reaction off gas is about 20 wt % or more. If the oxygen concentration in the reaction off gas is lower than about 20 wt %, the NO oxidation efficiency declines.
In the present invention, in the reaction product solution mainly composed of DMSO delivered from the oxidation reactor, the NOx which has not been released as the reaction off gas is dissolved by an amount corresponding to the solubility of NOx in DMSO. In the present invention, the dissolved NOx is removed from the reaction product solution mainly composed of DMSO, by blowing nitrogen gas into the reaction product solution. In this case, the preferable temperature of the reaction product solution into which nitrogen gas is blown is about 40xc2x0 C. to about 90xc2x0 C. A more preferable range is about 50xc2x0 C. to about 80xc2x0 C. It is preferable that the amount of the nitrogen to be blown in is about 1 wt % to about 7 wt % based on the weight of the reaction product solution. A more preferable range is about 2 wt % to about 6 wt %.
Furthermore, in the present invention, the reaction off gas and/or the gas removed from the reaction product solution respectively containing NOx is passed through a NOx absorbing column, using the reaction product solution remaining after removing NOx as a NOx absorbable solution, to recover NOx from the reaction off gas and/or the gas removed from the reaction product solution.
The absorption efficiency is not so greatly affected by the type of the NOx absorbing column, but a packed column, plate column or wetted wall column can be preferably used.
It is preferable that the amount of the reaction product solution mainly composed of DMSO remaining after removing NOx used as the absorbing solution of the absorbing column is about 5 times or more than the amount of NOx. It is more preferable that the amount is up to about 10 times or more.